Attention is currently being focused on fine cellulose fibers in which cellulose-based fibers are beaten and crushed at a high level to increase their fineness (fibrillate) to a fiber diameter of 1 μm or less. Sheets composed of fine cellulose fibers produced in a process consisting of forming a papermaking slurry of these fine cellulose fibers into paper followed by drying are expected to be applied to a wide range of applications in addition to conventional paper. For example, sheets controlled to be porous can be used as filters or membranes having extremely fine pores of 1 μm or less, enabling them to efficiently capture fine particles in a liquid or gas with low pressure loss, and can be used in fine filtration filters or virus removal filters and the like. Moreover, the pores of a porous sheet can be filled in with resin and compounded, enabling it to be used as a flexible, transparent resin sheet having a low coefficient of thermal expansion. On the other hand, transparent, highly dense sheets free of pores can be used as flexible electronic paper or gas barrier membranes and the like.
The water resistance of this fine cellulose fiber sheet is an important technical factor in terms of using in a wide range of applications. Since fine cellulose fiber sheets conventionally retain a sheet structure by hydrogen bonding between fine cellulose short fibers, the hydrogen bonds are easily cleaved and the sheet structure easily collapses as a result of contact with water. Thus, making these fine cellulose fiber sheets water resistant is essential for their use as water treatment filters and in other environments involving contact with water.
Two methods have been reported thus far as techniques used to enhance water resistance, consisting of i) post-processing and ii) internal addition of a water resistance agent.
The post-processing method is a method consisting of impregnating a deposited fine cellulose fiber sheet with an organic solvent containing a crosslinking agent followed by heat treatment. In the following Patent Document 1 according to the present applicant, water resistance is imparted by impregnating with a toluene solution containing 1,6′-hexamethylene diisocyanate or 4,4′-diphenylmethane diisocyanate. However, this technique is extremely disadvantageous from the viewpoints of industrial production and environmental issues since it uses an organic solvent. Even if a water-soluble or water-dispersible water resistance agent is used as a way of solving the aforementioned problems associated with organic solvents, there is an extremely high likelihood of the sheet tearing during treatment due to inadequate wet paper strength. Moreover, it is theoretically difficult to uniformly distribute the water resistance agent in the sheet, making this technique disadvantageous in terms of improving water resistance and other properties.
On the other hand, the internal addition method is a technique by which a water-soluble or water-dispersible water resistance agent is added to a cellulose fiber slurry to prepare a mixed liquid followed by forming into paper, drying and subjecting to heat treatment. In comparison with the post-processing method, the internal addition method is superior in terms of i) having fewer steps, (ii) not using an organic solvent, and (iii) allowing the water resistance agent to be more uniformly dispersed.
Patent Document 2 indicated below discloses a technique for imparting water resistance by using an aqueous emulsion of a polymer having a low glass transition point as a water resistance agent to form a polymer coating on the surface of fine cellulose fibers having a fiber width of 2 nm to 1000 nm followed by compounding with a resin.
In addition, Patent Document 3 indicated below discloses a technique for imparting water resistance in the production of paper using ordinary pulp in the form of needle bleached kraft pulp (NBKP) by using a thermal reaction type, water-soluble urethane prepolymer as a water resistance agent and chemically crosslinking cellulose fibers with the thermal reaction type, water-soluble urethane prepolymer. Differing from the polymer coating having a melting point of Patent Document 2, the use of chemical crosslinking makes it possible to prevent elution into organic solvent while also enabling use in an environment at a temperature equal to or higher than the melting point of the polymer.
Fine cellulose fiber sheets that contribute to industrial use are required to be sheets in which not only water resistance, but also various other properties and functions (such as papermaking ability, solvent resistance, adhesion, functionalization agent immobilization, surface zeta potential, hydrophilicity/hydrophobicity or air permeability resistance) are simultaneously controlled. However, although the aforementioned patent documents contain descriptions regarding imparting water resistance, there is no mention made regarding other properties and functions. Namely, there is currently no technology for producing a fine cellulose fiber sheet in which multiple properties and functions, including water resistance, are controlled.
Moreover, in the case of considering industrial productivity as well as environmental considerations that have come to be required in recent years, there has been a desire to achieve a technique for precisely controlling the aforementioned properties and functions that employs a labor-saving process while having a low impact on the environment.